PDCD1 and cancer: The two immune checkpoint receptor–ligand systems whose inhibition has proven to be successful in the cancer clinic are the cytotoxic T lymphocyte-associated protein 4 (CTLA4)–B7-1/B7-213 and the programmed cell death protein-1 (PD-1)–programmed cell death 1 ligand 1 (PD-L1) systems14.